1. Field of the Invention
The present invention relates to a dual-mode two-pole filter such as, for example a band pass filter for the microwave range, consisting of a resonator for two dipole modes each having a dielectric arranged on at least one high-temperature superconductive film, with a shielding housing arranged over the high-temperature superconductive film and enclosing the dielectric, and with a coupling device for coupling the dipole modes and in particular for coupling microwaves to the electromagnetic fields of the dipole modes employed. The invention, furthermore, relates to a multipole dual-mode filter.
2. Prior Art
In the satellite communications technology, band pass filters in the microwave range (4 to 20 GHz) play an important role in the preselection of individual communication channels. Analog multiplexers are usually employed there with filters based on hollow resonators, in connection with which the quality factors of the individual resonators are in the range of a few 104. The hollow resonators, which have a circular cylindrical shape in most cases, are mostly operated in so-called xe2x80x9cdual modesxe2x80x9d, i.e., orthogonal dipole modes with preferred direction of the electromagnetic fields in the circular area. This loads to the fact that two poles of a filter can be realized with one resonator, i.e., an n-pole filter based on dual-mode resonators consists of n/2 resonators.
Now, in satellite communications technology, miniaturization of the filters is important on the one hand, and reduction of the insertion attenuation is important mainly for the output multiplexers on the other. This reduction in insertion attenuation leads to the fact that it is possible to reduce the high-frequency output of the output stages, which is normally generated by tube amplifiers (travelling-wave tube amplifiers). The insertion attenuation decreases with the increasing quality factor of the individual resonators.
A highly miniaturized dual-mode filter based on dielectric resonators is known from U.S. Pat. No. 4,489,293, where use is made of the HE111-mode of a cylindrical dielectric, which splits into two orthogonal dipole modes.
Further miniaturization of the arrangement described in the above state of the art is achieved, for example in that the cylindrical dielectric resonator is divided in half parallel with its base area and placed on a film consisting of high-temperature superconductors. The volume of the resonator is divided in half thereby (image plane).
The drawback of these arrangements lies in the fact that in the HE111-mode employed, the loss contributions of the metallic shielding housing lead to quality factors which are only in the range of 104. The cause for this is the following:
The unstressed quality factor Q0 of a dielectric resonator with metallic shielding is given by the expression
1/Q0=tan xcex4+xcexa3Rs.i/Gi,
where tan xcex4 is the loss tangent of the material of the dielectric resonator. Some dielectrics such as sapphires, LaAlO3 and rutile have tanxcex4-values of a few 10xe2x88x926 or even less below a temperature of T=100 K, so that qualities between 105 and 106 would basically be possible with cooled dielectric resonators. However, the limitation is caused by losses in the various parts of the wall of the metallic shielding housing, such losses each being characterized by the surface resistance of the wall material RS.i as well as by a geometric factor Gi for the particular part xe2x80x9cixe2x80x9d of the wall. The geometric factor G; results from the distribution of the electromagnetic fields for the given mode of oscillation of the resonator. With the mode of oscillation employed in the aforementioned state of the art, the geometric factors are so low that the qualities for a copper housing with normal conductivity come to about 104. The superconductive xe2x80x9cimage planexe2x80x9d did not lead to higher qualities because the losses dominated in the remaining parts of the wall with normal conductivity, and also in the dielectric.
Circularly cylindrical dielectric resonators with two end plates made of high-temperature superconductive films are known from WO 93/09575. Qualities in the range of 106 were demonstrated with such resonators because the geometric factor is adequately high for the normally conductive jacket surface of the cylinder in the TE011-mode used there. However, due to the rotation symmetry of the filed distribution, the mode is not a dipole mode, so that no xe2x80x9cdual-modexe2x80x9d operation is possible in this case.
Therefore, the problem of the invention is to create a dual-mode filter in connection with which the quality factors for the individual resonators are about 105 to 106.
According to the invention, the problem is solved for a dipole filter in that the dielectric has the shape of a hemisphere. The realization of more than two-pole filters is solved according to an arrangement where the spacing of at least two hemispherically shaped dielectrics defines the coupling between two resonators.
By shaping the dielectric according to the invention, the curved surfaces of the shielding housing have geometric factors which are sufficiently high for obtaining with one or two high-temperature superconductive films (HTS-films) the qualities required in connection with the problem on hand.
Additional embodiments of the present invention contain advantageous features for coupling to the electromagnetic fields of the dipole modes for equalizing the resonance frequency of the dipole modes as well as for adjusting the coupling between the dipole modes.
Additional advantages of the present invention are discussed below.